(1) Field of the Invention
The present application relates to an over-current protection device, and more particularly to a surface-mountable over-current protection device.
(2) Description of the Related Art
Over-current protection devices are used for protecting circuitries from damage resulted from over-heat or over-current. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has positive temperature coefficient (PTC) characteristic that the resistance thereof remains extremely low at room temperature and instantaneously increases to thousand times when the temperature reaches a critical temperature or the circuit has over-current, so as to suppress over-current and protect the cell or the circuit device. When the resistive material gets back to the room temperature or over-current no longer exists, the over-current protection device returns to be of low resistance and as a consequence the circuitry again operate normally. In view of the reusable property, the PTC over-current protection devices can replace traditional fuses, and have been widely applied to high density circuits.
Referring to FIG. 1, U.S. Pat. No. 6,377,467 disclosed a surface mountable over-current protection device 10 containing a resistive device 11, a first electrode 17, a second electrode 18, insulating layers 15 and 16, a first conductive via 19 and a second conductive via 20. The resistive device 11 contains a first conductive member 13, a second conductive member 14 and a polymeric material layer 12. The polymeric material layer 12 is stacked between the first conductive member 13 and the second conductive member 14. The first electrode 17 comprises a pair of electrode foils disposed on the insulating layers 15 and 16, and is coupled to the first conductive via 19. The second electrode 18 comprises a pair of electrode foils disposed on the insulating layers 15 and 16, and is coupled to the second conductive via 20. The conductive vias 19 and 20 are approximately placed at the centers of two opposite planar lateral surfaces and are in the shape of semi-circular holes.
Electronic apparatuses are being developed with reducing size trend. Therefore, it is desirable to use small over-current protection devices. For example, the form factor of the devices has been advanced from 1210, 1206, 0805, 0603, 0402 to 0201. However, the conductive vias 19 and 20 in the form of semi-circular holes will encounter manufacturing problems when the devices have been shrunk to below 0603.
Referring to FIGS. 2A and 2B, normally, the smaller the devices, the smaller the hole sizes of the conductive vias 19 and 20 are. For example, the radius of the semi-circular conductive via is around 0.15 mm for a 0603-type device. In the manufacturing process of cutting to form devices, the width “d” of the cutter has to be aligned with the cutting line (see FIG. 2A). However, if the cutter is misaligned, it is likely to cut off a large amount of a conductive via of one of the adjacent devices, or even remove the entire conductive via (see FIG. 2B). Smaller devices have to have large semi-circular conductive vias to avoid the problem. Nevertheless, large conductive vias further generate other manufacturing process issues as mentioned below.
When the over-current protection devices are subjected to appearance inspection, resistance measurement or packaging process, they are pushed one-by-one in a track 24. If the devices 10 have smaller semi-circular conductive vias, they could be smoothly pushed to go forward, as shown in FIG. 3A. If the devices 10 have larger semi-circular vias especially the ones of which the concave semi-circular conductive via having a width greater than half the side width of the device 10, the protrusions of a device 10 would be engaged with the concave semi-circular conductive via of the one next to it, resulting in unstable transmission or blockage of the devices 10, as shown in FIG. 3B.
As the advancement of the devices of 0603, 0402 or even smaller type is vital for new applications, it is highly demanded to have a solution on how to obviate the manufacturing issues mentioned above.